This invention relates to an art for taking a reliable safety measure when an anomaly occurs in a discharge lamp lighting circuit.
The configuration of a lighting circuit of a discharge lamp, such as a metal halide lamp, comprising a DC power supply circuit, a DC-AC conversion circuit, and a starter circuit is known. For example, when an anomaly occurs in the discharge lamp or the lighting circuit, an anomaly detection circuit or a control circuit detects the anomaly and stops the operation of the lighting circuit, so that a high voltage can be prevented from injuring a human body or the detrimental effects of smoking, firing, etc., caused by excessive electric power output can be prevented.
However, if the discharge lamp is simply turned off, the safety measure is insufficient in application to vehicle lamps, etc., because there is a problem of requiring the driver to run the vehicle in the dark, etc. Measures of lighting an alternative light source, notifying the driver that an anomaly occurs, etc., are taken.
However, in the above-described measures, if an anomaly occurs in the circuit itself provided for detecting an anomaly, lighting the alternative light source, or notifying the driver that an anomaly occurs, there are possibilities that the intended safety measure may not be taken.
For example, when the control circuit for controlling lighting of the discharge lamp and detecting an anomaly occurring fails, etc., it becomes impossible not only to light the discharge lamp, but also to light the alternative light source, etc.
It is therefore an object of the invention to take a sufficient safety measure before an anomaly occurs in control of a discharge lamp or a lighting circuit and causes a serious condition.
According to the invention, there is provided a discharge lamp lighting circuit comprising a control circuit for detecting an anomaly occurring in a discharge lamp or the lighting circuit, stopping power supply to the discharge lamp or stopping operation of the lighting circuit, and lighting another light source as an alternative light source to the discharge lamp or notifying a driver of occurrence of the anomaly, wherein an internal state monitoring circuit for monitoring the operation state of the control circuit itself is provided and wherein when the internal state monitoring circuit detects an anomaly in the control circuit or detects a sign of occurrence of an anomaly, the alternative light source is lighted or the driver is notified of anomaly occurrence.
Thus, according to the invention, the internal state monitoring circuit monitors the operation state of the control circuit, whereby the alternative light source is lighted or the driver is notified of anomaly occurrence before the control circuit shows abnormal operation.